1. Field of the Invention
The present invention relates to methods and connections for securing together the ends of threaded tubular bodies to produce a structurally strong and leak resistant joinder of such tubular bodies. More specifically, the present invention relates to a threaded coupling and pin assembly, and associated method, for securing together the threaded ends of tubular pipe bodies such that the engaged pin and coupling threads will form a secure mechanical engagement and a high-pressure, leak resistant thread seal.
2. Background Setting of the Invention
Tubular pipe bodies used in the construction of oil and gas wells are typically secured together at their ends by threaded connectors that employ a thread and seal design intended to withstand the structural forces acting between the threaded components as well as to prevent the flow of high-pressure well fluids through the engaged threads of the connection. In many of the thread designs used to secure the pipe ends together, the threads function both as mechanical engagement structures and as pressure sealing structures.
Connections in which sealing is not an objective of the thread design often achieve sealing with supplementary seal rings and/or metal-to-metal seals that are constructed as integral components of the connection. Connections incorporating these seal enhancing features are often referred to as “premium connections”. Premium connection designs can require the use of couplings with coupling walls thicker than those of the coupling stock used to make couplings meeting specifications of the American Petroleum Institute (API). These heavier walled couplings are required to provide torque shoulders and to maintain a high coupling rigidity that aides in accommodating stress in the coupling during extreme internal/external pressures or tensile/compressive loads. When conditions permit, it is usually desirable to avoid as many as possible of the added features of these premium connectors, such as seal rings, metal-to-metal seals and heavy wall coupling stock because of the added cost and complexity associated with their use.
The coupling stock having dimensions and specifications such as from which API couplings are made is herein referred to as “standard coupling stock”. Standard coupling stock typically has a thinner wall construction and is more readily available than that required for many premium connections. A connection that may be built using standard coupling stock is less expensive to manufacture than one requiring coupling stock with a wall thickness greater than that found in standard coupling stock. Even within the range of standard coupling stock, the cost savings may be maximized by using the thinnest wall standard coupling stock possible because of the difference in steel content.
The following table lists the specifications for “standard coupling stock” used in the construction of API couplings for typical pipe sizes:
Standard Coupling Stock for CasingNominal OD, InchesCouplingNom. WallWt/ftCasingStockThread Type(Inches)(lbs)FBS 4½5.000Short.46622.594.185Long.47923.154.162Buttress.44321.584.225 55.563Short.50727.404.675Long.52928.474.638Buttress.48326.234.718 5½6.050Short.50029.665.175Long.52631.065.130Buttress.47728.425.215 6⅝7.390Short.63545.856.278Long.66247.616.232Buttress.60744.016.328 77.656Short.57343.396.653Long.60345.466.600Buttress.55241.926.690 7⅝8.500Short.70658.827.265Long.73561.017.214Buttress.68357.077.305 8⅝9.625Short.78173.848.258Long.82677.698.180Buttress.76072.028.295 9⅝10.625Short.78182.199.258Long.82986.819.175Buttress.76080.159.29510¾11.750Short.78692.1210.375Buttress.75989.1810.42211¾12.750Short.786100.5311.375Buttress.76397.7711.41513⅜14.375Short.786114.1813.000Buttress.760110.6113.0451617.000Short.803139.0415.594Buttress.833143.9615.54318⅝20.000Short1.018206.5718.219Buttress1.048212.3218.1662021.000Short.803173.3719.594Buttress.834179.7919.541
In addition to being more costly, thick wall coupling designs reduce the central clearance through the connector. The nominal outside diameter of all coupling stock is the same for any given pipe size to be used in a conventional string design. Coupling strength may be obtained by increasing the wall thickness toward the center of the coupling, however such strengthening results in reducing the clearance through the coupling. Reduced coupling clearance is usually associated with an undesirable reduction in the clearance through the pipe engaged in the coupling.
Recently, the industry has required that threaded pipe connections meet certain new performance and testing criteria. As a part of this requirement, the connectors are subjected to rigorous testing for qualification to new industry standards. One such industry standard is the International Organization for Standardization (ISO) 13679 specification. ISO 13679 is an international specification outlining the procedures to be used in testing casing and tubing connections for the oil and natural gas industries. This specification was developed to more realistically validate performance parameters by testing extreme service conditions and loads that the tubing and casing connections see during use. The ISO 13679 specification results, in part, from a determination that design changes in existing connections may be required to attain useful service load envelopes for connectors. One aspect of the specification requires that the connection be assembled in such a way that at the final make-up, there is a preloading of the connector axially. It has been found that this axial preload can be advantageous in improving pressure capacity as the axial load on the connector goes from tension to compression.
The torque force applying the pre-load is frequently referred to as a “delta torque”. Delta torque may be defined as that part of the final make-up torque that is induced into axially interfering components of the pin and box connection after resolving the radial thread interference occurring during the make-up. FIG. 12 of the drawings illustrates a Torque (T) vs. Turns (TN) chart defining shouldering torque (ST), delta torque (DT) and final torque (FT). Torque T is plotted on the vertical axis and the amount of pin turning TN relative to the coupling is plotted on the horizontal axis. The maximum radial interference experienced in the make-up before engagement of shoulders is commonly referred to as the shouldering torque ST. The lower bound of the delta torque, i.e. the shoulder torque, is attributable to the thread interference that occurs before the connection is stopped from rotating because of the engagement of axially interfering connection components in the pin and box. The upper bound of delta torque, or final torque, is the total torque applied to the connection during make-up. It will be understood that the magnitude of the pre-load force is a function of the magnitude of the delta torque. Increasing delta torque may be achieved by lowering the shoulder torque and/or increasing the final torque of a connection.
In general, preloading a connection requires that the connection be assembled with a high torque force stored in the connection. The upper boundary on torque is usually determined by the capacity of the hydraulic power tong that is used to apply the torque to the connection at the well site. The available make-up torque at the well site must be distributed to achieve an optimum balance of radial and axial forces within the connection. This requires imposing radial interference levels in the connection that are sufficient to energize the threads for sealing while conserving tong torque capacity for applying the required final torque to achieve a desired axial preload.
When axial compression loads are applied to the connection, either during testing or during use in the well, it is possible that the axial preload force will be exceeded. When this occurs, a cascading “micro-movement” effect occurs down the threads as load flanks are disengaged and stab flank clearances are reduced and eventually engaged. This micro-movement disturbs the thread compound in conventional thread designs and can lead to the initiation of leaks. Increasing the final torque is not necessarily an economical option for increasing the preload in an attempt to overcome this susceptibility to leakage. In addition to the fact that the rig tongs may often lack capacity to add sufficient final torque, very high torque values applied to the pipe increase the opportunity to damage the pipe body and/or the pipe threads.
The prior art has suggested a variety of different assembly methods and thread designs in an effort to prevent leakage in connectors that are subjected to high axial tension and compressive loads. In addition to preloading the connection, the prior art has suggested specific connection thread configurations for improving the seal between engaged threaded connectors. The prior art specifically teaches thread designs that employ thread seals for sealing internal and external pressure in an environment subjected to axial tension and compression loads. Certain of these prior art designs feature negative load flank threads, diametrical (radial) interference levels, torque shoulders and special thread clearances to enhance a thread seal. Biased tapers between the pin and box to aid in the seal created by the thread compound-to-thread profile interface are also features of prior art connections. Designs incorporating a torque shoulder that is achieve by abutting the pin noses of the two pipe joints engaged within a coupling have also been proposed. Some of the prior art designs have also combined abutting pin nose shoulders with one or more of the other seal enhancing features known in the prior art designs. The benefits of constructing a connection within the limits of readily available and economic American Petroleum Institute (API) coupling stock are also well recognized by prior art designers. None of the prior art, however, teaches or suggests an easily manufactured, economical connection that provides a satisfactory thread seal in conventional coupling stock within an environment subjected to high axial stress loads with final make-up torques that are within the working capacity of the power tongs commonly used to assemble connections at the well site.